This application claims the benefit of Japanese Patent Applications No. 2002-086905 filed Mar. 26, 2002, No. 2002-099540 filed Apr. 2, 2002, and No. 2003-042721 filed Feb. 20, 2003, in the Japanese Patent Office, the disclosure of which is hereby incorporated by reference.
1. Field of the Invention
The present invention generally relates to imaging units, optical write units, optical read units and image forming apparatuses, and more particularly to an imaging unit, an optical read unit and an image forming apparatus which use an optical path shifting element, and to an optical write unit and an image forming apparatus which use a light emitting element array.
2. Description of the Related Art
There are various kinds of image forming apparatuses, such as electrophotography type copying machines, printers, facsimile machines and composite apparatuses (MFPs) having multiple functions such as copying, printing and facsimile functions, photoexposure type apparatuses and printers which use photosensitive materials, and printers and facsimile machines which use thermal materials. In such image forming apparatuses, an image forming engine may be formed by an optical write unit which uses a light emitting element array.
In the optical write unit which uses the light emitting element array, it is necessary to reduce the pitch of the light emitting elements of the light emitting element array, in order to increase the resolution of the printed image. For example, when producing a light emitting element array having a pixel pitch of 1200 dpi, the pitch of the light emitting elements becomes approximately 21 μm. However, the cost of such a light emitting element array becomes high, because of the need to employ a high-density mounting technique such as wire-bonding.
Accordingly, an optical write unit was previously proposed in a Japanese Laid-Open Patent Application No. 8-118726, which uses a light emitting element array having a low resolution but enables printing at a high resolution. This proposed optical write unit uses an imaging position control means for electrooptically changing an exposure position, by combining a ferroelectric liquid crystal cell which rotates a plane of polarization by 90 degrees and a birefringence plate.
FIG. 1 is a cross sectional view showing an example of a conventional optical write unit, such as that proposed in the Japanese Laid-Open Patent Application No. 8-118726.
In FIG. 1, a transparent electrode 2 and a horizontal alignment layer 3 are formed on each of a pair of transparent substrates 1. A liquid crystal layer 4 made of a chiral smectic C phase ferroelectric liquid crystal is sandwiched between the horizontal alignment layers 3 provided on the pair of transparent substrates 1. The thickness of the liquid crystal layer 4 is set smaller than the spiral pitch of the chiral smectic C phase. Hence, the liquid crystal layer 4 forms a surface stabilizing type ferroelectric liquid crystal cell.
The liquid crystal layer 4 is made of a liquid crystal material which makes a 45 degree change in the alignment direction of the liquid crystal directors when the electric field is switched. Hence, it is possible to rotate the plane of polarization by 90 degrees by performing the switching so that the direction of the liquid crystal directors becomes parallel to or 45 degrees to the plane of linearly polarized light of the incident light.
In addition, by providing the birefringence plate 5 at a stage subsequent to the liquid crystal cell formed by the liquid crystal layer 4, the light propagates linearly when the plane of polarization is such that the light becomes an ordinary ray component with respect to the birefringence plate 5, and the light makes a parallel shift when the plane of polarization is such that the light becomes an extraordinary ray component with respect to the birefringence plate 5. In this case, the amount of shift of the optical path is determined by the direction of the optical axis of the birefringence plate 5 and the thickness of the birefringence plate 5.
The optical write unit is formed by interposing an optical path shifting element 6 having the structure described above between a light emitting element array 7 and a recording medium 8. In this optical write unit, light emitting from a plurality of light emitting elements 9 of the light emitting element array 7 is converged by a lens (not shown), and is thereafter irradiated on the recording medium 8 via the optical path shifting element 6. By switching the polarity of the electric field applied to the liquid crystal layer 4 via the transparent electrodes 2, the direction of the spontaneous polarization of the liquid crystal layer 4 switches between the directions indicated by symbols “●” and “{circle around (X)}” shown in FIG. 1. Hence, the light emitted from the light emitting element array 7 exposes the recording medium 8 at a pitch which is ½ the pitch of the light emitting elements 9, depending on the operation of switching the spontaneous polarization direction by the optical path shifting element 6. In other words, if the light emitting elements 9 are arranged at the pitch of N μm, it is possible to expose the recording medium 8 by shifting the optical path in the direction in which the light emitting elements 9 are arranged by N/2 μm.
Therefore, by using the optical write unit described above in the image forming apparatus, it is possible to print at a high resolution even by use of the light emitting element array having a lower resolution. Moreover, the surface stabilizing type ferroelectric liquid crystal cell formed by the liquid crystal layer 4 can be switched at a high speed, so that a high-speed optical path shifting can be realized.
However, the optical write unit shown in FIG. 1 has the following problems (1)-(5).
(1) The surface stabilizing type ferroelectric liquid crystal cell requires the cell gap to be controlled with a high accuracy, and for this reason, it is difficult to produce the surface stabilizing type ferroelectric crystal cell within an area corresponding to the size of the light emitting element array.
(2) The surface stabilizing type ferroelectric liquid crystal cell requires a pair of transparent electrodes in the optical path, thereby decreasing the transmittance of the surface stabilizing type ferroelectric liquid crystal cell.
(3) An optical crystal which functions as the birefringence plate is expensive in general, and the cost of the optical write unit becomes high when the optical crystal covering an area corresponding to the size of the light emitting element array is used.
(4) Because the switching is made between the optical path linearly propagating through the birefringence plate and the optical path propagating obliquely through the birefringence plate, the focal point shifts due to the difference between the optical path lengths of the two optical paths.
(5) Since the amount of light propagation in the optical path is determined by the birefringence and the thickness of the optical crystal, the amount of light propagation in the optical path becomes fixed.
On the other hand, in an optical read unit used in digital copying machines, image scanners and the like, a document image is read by driving first and second scanning bodies to scan the document. The scanned document image is imaged by an imaging lens on a solid state image sensing device using CCDs.
In such an optical read unit, when the number of pixels is increased, in conveniences such as inconsistencies in the sensitivities with respect to each of the pixels, warping of the solid state image sensing device itself, increase of the production cost, the increase in the size of the optical read unit, and increase of the frequency of a read pixel clock for the solid state image sensing device occur. Hence, it is desirable to increase the reading density and to realize a high picture quality without introducing the above described inconveniences, and to suppress the so-called moire image without deteriorating the resolution characteristic. A method of realizing these desires shifts the solid state image sensing device in the main scan direction, and improves the reading density by reading between the light receiving parts of the solid state image sensing device for the adjacent pixels. Such a method is referred to as a pixel shifting method, and is sometime also referred to as a pixel shifting method or an image shifting method.
However, when combining the two images which are read by shifting the pixel position, the number of scans required increases because of the need to make the scan before and after the shifting of the solid state image sensing device. In addition, when reading while shifting the pixel position during one scan operation, it is necessary to shift the pixel position at a high speed. Furthermore, in order to shift the pixel position, it is necessary to move the solid state image sensing device itself by use a piezoelectric element or the like or, to shift the optical path by rotating the angle of an optical part such as a glass plate. In either case, it is necessary to use mechanical parts for the purpose of shifting the pixel position. For this reason, the mechanical structure of the optical read unit becomes complex, and vibrations and noise generated by the mechanical parts and the mechanical durability and reliability of the mechanical parts become a problem particularly when the pixel position is shifted at a high speed, as in the case of the mechanical structure proposed in a Japanese Laid-Open Patent Application No. 9-116704, for example.
Accordingly, a method of electrooptically shifting the optical path has been proposed. For example, a Japanese Laid-Open Patent Application No. 5-344431 proposes a combination of a birefringence plate and a liquid crystal element which rotates the plane of polarization.
In the optical read unit which uses the combination of the liquid crystal element and the birefringence plate, it is conceivable to use a surface stabilizing type ferroelectric liquid crystal cell as the liquid crystal element in order to realize a high-speed response.
In other words, a liquid crystal material which makes a 45 degree change in the alignment direction of the liquid crystal directors when the electric field is switched is used, and it is possible to rotate the plane of polarization by 90 degrees by performing the switching so that the direction of the liquid crystal directors becomes parallel to or 45 degrees to the plane of linearly polarized light of the incident light. In addition, by providing the birefringence plate at a stage subsequent to the liquid crystal cell, the light propagates linearly when the plane of polarization is such that the light becomes an ordinary ray component with respect to the birefringence plate, and the light makes a parallel shift when the plane of polarization is such that the light becomes an extraordinary ray component with respect to the birefringence plate. In this case, the amount of shift of the optical path is determined by the direction of the optical axis of the birefringence plate and the thickness of the birefringence plate. The ferroelectric liquid crystal cell is capable of making a high-speed switching, and thus, the optical path can be shifted at a high speed.
However, the optical read unit described above also has the problems (1)-(5) of the optical write unit described above.